This invention relates to a batch-fabricated silicon capacitive accelerometer. More particularly, this invention relates to such a capacitive transducer which utilizes a sandwich-type structure wherein a movable plate is mounted to move in a piston-like fashion in a cavity toward and away from a fixed plate to provide an exceptionally large area of movement of the movable plate in its movements toward and away from the fixed plate. That is, the entire area of the movable plate moves toward and away from the fixed plate, rather than in a flexing type pattern of movement usually provided with capacitive transducers of this type.
With the use of computers allowing increasingly complex instrumentation systems, there is a pressing need for individual sensors of high reliability and modest cost. Included in this need is a need for acceleration sensors, both to measure acceleration directly and to allow integration of acceleration and time to measure velocity and displacement. This is particularly true, as mentioned above, in manufacturing processes utilizing robot structures for the production of such items as automobiles. It is also important in propulsion systems for aircraft, for example, or projectiles of various kinds used in the armed services. It is important that such structures be extremely small, but still operate, as will be appreciated by practitioners-in-the-art, under extreme conditions of temperature and pressure vibration and shock and that they operate in an extremely sensitive fashion to applied stimuli over a very wide range.
The use of a rigid central "piston" moving on a flexure within a frame has already been developed. For example, such a structure is taught and claimed in U.S. Pat. No. 4,236,137. In that arrangement, the means for detecting motion is strain gages on the flexures on which the central plate flexes. By contrast, with this invention, the flexure is approximately in the center of the thickness of the plate. The central flexure is important because it prevents tilting of the central mass in response to accelerations in the plane of the plate during the sensing process. This is brought about in the process of the invention by controlling closely the depth of an etch. That is, the bottom of an etched cavity is modified, then released by another etch to form the flexible membrane, upon which the "piston-like" plate moves. With the device described in U.S. Pat. No. 4,236,137, gas damping has used the plane space between the plates of changing proximity with squeeze-film damping between the plates which is extremely dependent upon the proximity of the plates and is limited in its frequency range by the velocity of sound of the medium.
With this invention by contrast, a uniquely devised plate is produced utilizing a plurality of spaced apart holes or passages for movement of fluid therethrough. Thus, the fluid moves through the passages as the plate moves back and forth in its piston-like fashion. More importantly, special grooves are developed on the surfaces of the plate which have the effect of precisely directing the flow of fluid toward and away from the passages, thus guiding the fluid movement during movement of the piston-like movable plate. The principle gas flow resistance is in the spreading channels on the face of the plate. For this reason, damping in the perforations or holes in the plate avoids these problems.
Another feature of the invention here is the use of spots of glass as stops over the surface of the movable plate. In an overload, therefore, the stops prevent electrical shorting of the plates. The spots, in addition, prevent two forms of latch-up in an overload. In very sensitive capacitive sensors, the attractive force of the electrical bias or carrier will exceed the spring force of the membrane mounting the movable piston at some proximity in the movement of the piston-like plate toward the fixed plate. Therefore, the plates latch together. The height of the stop spots, in accordance with this invention, is chosen to prevent latch-up. For example, in one embodiment of the invention, the height is chosen to prevent latch-up at 5 volts with a restoring stiffness allowing for one micro inch travel in response to 1G acceleration.
Another latch-up problem can take the form of pneumatic latch-up. That is, as flat plates are brought very close into proximity with each other, the flow resistance into the space between them becomes very high. It may take a long time (several seconds) for gas to re-enter this space in a sensitive capacitive sensor, and allow the moving plate to return to its normal position after an overload. The stop spots of the invention spaced over the surface of the plate keep the space open between the two plates to allow gas flow and rapid recovery.
As stated above, one of the features of the invention here is the mounting of the movable plate in a sandwich structure wherein a cavity is formed between the two outer layers with a central layer or core being the structure mounting the movable capacitor plate, and with the core portion or layer also forming part of the cavity on the plate surface on either side. Thus, the formed cavity has communication on either side of the movable plate through the perforations formed in the plate.
An additional feature of the invention is the use of small silicon knobs, ridges, bars or other protuberances on the core which are jammed into an aluminum film on an opposing part of the sandwich, which has the effect that the knobs are held in contact by elastic deformation of the underlying material which makes for a stable connection between the parts. Finally, in the processing of the capacitor plate of the invention, highly sensitive capacitive sensors of very small size are produced, in accordance herewith, with uniform thin flexures by means of a diffused etch stop.
In considering generally the conditions for producing a silicon capacitive sensor in accordance with this invention, it is important to realize that the resulting instrument must provide a capacitor gap which is controllable within .+-.10%, and the residual membrane thickness must be controllable within .+-.8%. Also, the residual membrane must be substantially at the mid plane of the wafer. Furthermore, the width of the channels on the face must be controllable within .+-.15% of minimum dimension and have predictable rounding at channel exits, while there must be, of course, the throughholes. In addition, the throughholes and channels must not have fragile overhanging material. The stop spots must be insulated to withstand anodic bonding voltages, and be within 0.6 and 1.0 microns in thickness. Finally, the bonding rims must be flat and smooth for anodic bonding, and no photolithography is allowed after the membranes are revealed or developed.
As purely illustrative of a process for achieving the desired results, and particularly in the production of the central movable plate structure of the sandwich structure of the invention, the starting material is a single crystal silicon wafer N or P type (100) plane indexed [110] within 0.7.degree.. The initial thickness is selected to be 0.0075 inches .+-.0.0002 inches. Both sides are polished and then oxidized lightly to about 0.3 microns thickness. Thereafter photolithograph matched index patterns are applied to the front and back oxide coats and the open index spots are reoxidized. Then, photolithographic patterns of its grooves, channels and throughholes are opened in the oxide on the front surface, and throughholes in the oxide on the back surfaces. The wafer is then etched in a potassium hydroxide etch slightly more than halfway through (0.0039 inches) to open the throughholes. Subsequently, the front surface is photolithographed, removing the remaining oxide from the central movable portion, leaving the rims in tact. Thereafter, using a precise shallow etch (i.e., ion beam milling) 3.5 microns .+-.10% are removed from the exposed silicon of the front surface, thus forming the capacitor gap.
Following this, all remaining oxide is stripped, and both sides reoxidized with a heavy oxide layer to thickness of about 0.6 microns. The back surface is photolithographed to leave oxide only over areas to be attacked in final etch, and remove all other oxide. All exposed silicon is then doped with boron to the extent wherein for a depth of 2.2 microns the boron concentration is in excess of 5.times.10.sup.19 atom/cc.
In a dry O.sub.2 environment, the wafer is reoxidized to reduce to 1.6 microns the net depth in which boron exceeds 5.times.10.sup.19 atoms/cc. The layer of high doping will taper, in this situation, into lower doping both inward and outward. The oxide should be about 0.6 microns. Photolithography is then applied to leave stop spots of oxide on the face, followed by removal of all other oxide. Finally, the final etchant is applied (ethylene diamine/ pyrocatechol [EDP] etchant) to reveal residual membranes of silicon doped more than 5.times.10.sup.19 atom/cc.
In forming the lid and base of the sandwich structure of the invention, one procedure is to provide the lid and base of Pyrex. In this connection, a borosilicate glass is used and preferably Pyrex. The requirements for the glass are that it must have a thermal expansion close to silicon, so that during cooling, after bonding of the two surfaces together, contraction is closely related to silicon. Moreover, the glass should have an electrical conductivity, at the bonding temperature of 450.degree.-550.degree. C., of between 10.sup.5 and 10.sup.8 ohm Cm. Finally, the glass should have mechanical and chemical stability at temperatures below 100.degree. C. The base plate carries the stationary plate of the capacitor. This may be a thin metallic film opposite the moving plate, with an extension through a notch in the rim of the central part, for electrical connection. This notch may be sealed with solderglass to assure cleanliness of the interior of the device. The lid must have a recess opposite the moving portion of the central part to allow motion. These two Pyrex parts can be anodically bonded to the central part.
Alternatively, the lid and base may be principally silicon, with inlays of Pyrex or equivalent to provide insulating, low capacitance bonds between layers. One procedure for making such composites of Pyrex and silicon is to etch into the silicon, recesses slightly deeper than the desired thickness of Pyrex inlay. Pyrex frit is then deposited on the silicon by sedimentation, and fused to form a solid layer of glass with a thickness greater than the depth recesses.
The surface of the wafer is then ground and polished to remove the Pyrex from the undisturbed surface of the silicon to produce a flat, smooth surface on the Pyrex in the recesses. The base, which is the fixed plate of the capacitor, is coated with a thin aluminum film, which is patterned photolithographically to provide convenient connections, both to the base and the central layer with the movable plate. The lid requires a recess to permit motion of the moving plate. Such recess is conveniently made by etching in the EDP etchant, which attacks the silicon but not the Pyrex.
There are two objects in using silicon outside layers with Pyrex inlays. One is to go out-of-plane with the fixed electrode to remove the need for a notch in the bonding rim and the need then to plug that notch. The other is to reduce the dissimilarity of thermal expansion among the parts by making them more nearly all one material.
With the foregoing and additional objects in view, this invention will now be described in more detail and other objects and advantages hereof will be apparent from the following description, the accompanying drawings, and the appended claims.